Problems associated with electromagnetic interference (EMI) are commonly encountered in the field of electronics. EMI is of particular interest in networking, telecommunications, and data-processing systems. Electronic components of such systems generate electromagnetic fields that interfere with the operation of other internal components, often causing malfunction thereof. Similarly, EMI produced by electronic devices, e.g., computers, adversely affects the performance of neighboring appliances, such as printers, telephones, etc.
Electronics systems may comprise a conductive chassis that houses a plurality of printed circuit (PC) boards, otherwise known as circuit cards. Electrical-current discontinuities, or gaps, between the cards and the chassis allow electromagnetic radiation to emanate, thus producing EMI. However, it is known in the art that electromagnetic radiation of a particular frequency is attenuated as the size of the discontinuities is reduced below half the radiation wavelength. Therefore, EMI can be minimized by properly grounding the cards to the chassis.
Electrical connections between the conductive chassis and the circuit cards are necessary also because the chassis is typically utilized as an electrical ground required for the operation of individual PC boards. Because of the high density of electronic components mounted on a typical PC board, most boards require a plurality of independent grounding points to fully satisfy their grounding needs.
Currently, several types of grounding mechanisms are known and utilized. For example, grounding guides that allow a circuit card to be slidingly installed into the chassis are manufactured by Unitrack Industries, Inc., West Chester, Pa.
As shown in FIG. 1, each of these guides comprises a conductive strip 20 that includes two corresponding pluralities of paired resilient fingers 22 and 24, evenly spaced along the strip. Strip 20 is inserted into a conductive track 26 (FIG. 2) having a longitudinal slot 27 and a flange 28 formed so that fingers 22 engage the flange to produce electrical continuity between strip 20 and track 26. Tabs 30 and 32 are provided at each end of track 26 for attaching the card guide to a chassis (not shown) and for retaining strip 20 inside the track.
As an edge of a circuit card (not shown) is inserted into slot 27, it slides between flange 28 and fingers 24. To accommodate the card, the resilient fingers are displaced laterally. Thus, the circuit card is electrically interconnected with strip 20 via a plurality of fingers 24, each having a contact patch formed as a point contact. Similarly, the strip has a corresponding number of electrical connections, i.e., fingers 22, with track 26. In turn, the track is grounded with screw-type fasteners (not shown), anchoring tabs 30 and 32 to the chassis.
However, the above-described card guide possesses a number of salient flaws. Specifically, the guide provides insufficient grounding of the circuit card to the chassis since the former is grounded to the latter only by two fasteners that attach tabs 30 and 32 to the chassis. Thus, an electrical discontinuity is created between the card and the chassis along the length of the track, which separates the two fasteners. Such a discontinuity allows generation of EMI.
Additionally, the card is grounded to the chassis indirectly because the resilient fingers of the conductive strip electrically couple the card only to the track, whereas the track is then grounded to the chassis by means of mounting tabs and fasteners. Therefore, even if additional grounding points were provided between the track and the chassis, a direct electrical connection between the card and the chassis still would not be established. Moreover, introduction of such grounding points in the form of rivets or screws significantly increases the manufacturing costs.
Furthermore, since the guides are rarely removed from the chassis, track-to-chassis attachment points are subject to corrosion that causes deterioration of the grounding connections. Similarly, because fingers 22 are fixed with respect to flange 28, corrosion may also produce the loss of electrical continuity between the fingers and the flange.
Although corrosion between the card and the guide is insignificant since the card is periodically removed from and replaced into the guides, the prior-art guides present a different set of problems. As the card is repeatedly inserted into and extracted from the guides, material is removed from the face of the card because the point contacts of fingers 22 apply relatively large forces over a small area of the card. Hence, excessive galling produced along one face of the card is another drawback associated with the prior-art guide. The resulting metal debris may contaminate electronic components located within the chassis, causing them to malfunction. Moreover, the high pressure exerted by the resilient fingers on the face of the circuit card increases the possibility of damage to the contacts due to snagging. Also, as the face of the card becomes abraded, the coefficient of friction between fingers 22 and the card increases, making it difficult to slide the card in and out of the guide. Furthermore, the circuit card is not inherently centered between its respective guides. The lack of centering may hinder positive engagement between the connectors located on the leading edge of the card and the mating connectors situated on the back plane of the conductive chassis.
Additionally, the insertion of the circuit card into the guides is cumbersome since the track slots are relatively narrow and aligning both edges of the card with the slots of the corresponding tracks requires increased time and effort.